In the sewn goods industry, where various sections of material are sewn together to fabricate products, reasonably precise seam lengths and end points are often necessary for proper appearance and function of the finished products. For example, the top stitch seam of a shirt collar must closely follow the contour of the collar and terminate at a precise point which matches the opposite collar. Accurate seam lengths must similarly be maintained in the construction of shoes when sewing together vamps and quarter pieces to achieve strength as well as pleasing appearance. Achieving consistently accurate seam lengths and end points at high rates of production has, however, been a long standing problem in the industry.
Microprocessor controllers have been developed which convert manually operated sewing machines into semi-automatic sewing systems. U.S. Pat. Nos. 4,108,090; 4,104,976; 4,100,865; and 4,092,937, assigned to the Singer Company are representative of such devices. Each of those patents discloses a programmable sewing machine with three operational modes: manual, teach and auto. Control parameters are programmed into the system for subsequent control of the sewing machine in the auto mode. Those microprocessors control all sewing machine functions such as sewing speed, presser foot position, thread trimmer, reverse sew mechanism and the number of stitches sewn in each individual seam. They thus offer higher speeds of operation and consequently higher rates of productivity than do manually operated sewing machines. Accurate control of seam lengths is one of the important aspects of those systems.
U.S. Pat. No. 4,404,919, issued Sept. 20, 1983, entitled "Control System for Providing Stitch Length Control of a Sewing Machine", and assigned to assignor describes a microprocessor controlled sewing system which improves upon the seam length accuracy of those systems. The system disclosed in U.S. Pat. No. 4,404,919 controls seam length accuracy using a combination of stitch counting, edge detection and stitch length control techniques. Control of seam lengths and end points is achieved in the system by initiating countdown of a variable number of final whole and partial stitches responsive to detection of the edge of the material being sewn by sensors located ahead of the needle. In dependence upon the amount of the stitch which has been sewn upon edge detection by the sensors, the microprocessor issues a signal to position the reverse sew mechanism of the sewing machine while the last stitch is being formed to reduce the length of the last stitch to a desired percentage of the normal stitch length and thus improves the accuracy of the seam end point.
An important feature of the disclosed system is the ability of the sensors to detect the presense of material being sewn and to signal the approach of the seam end. The sensors are used as external edge sensors to control the end of the seam by detecting the presence or absence of material at the boundary of the bottom ply of material being sew, as when sewing a collar seam. Alternatively, the sensors may be used as internal edge sensors to detect the difference between "N" plies and "N+M" plies of material, as when sewing a patch pocket onto the front panel of a shirt or a pants panel. In the latter example, the sensors signal the end of a pocket seam by detecting the difference between two plies of material, i.e., the pocket and the panel on which it is being sewn, and the single ply of material forming the panel.
Prior microprocessor controller systems have commonly used manually variable sensitivity sensors as internal edge detectors. When such sensors are used, the sensitivity of the sensor must be manually adjusted to ensure that the sensor detects the boundary of the relevant ply of material. In a commercial environment where a wide variety of materials are sewn in a single bundle of production work, the sewing machine operator using such prior systems must frequently readjust the sensitivity of the sensor to compensate for material variances, such as thickness, color, construction and finish and for the number of plies of material being sewn. The time required for such sensor sensitivity adjustment reduces the operator's production rate and the benefits attendant with use of microprocessor controlled systems.
A need has thus arisen for an improved adaptive sewing machine control system which includes automatic means for dynamically adjusting the sensitivity of the edge sensors to compensate for material variances to thus obtain more accurate seam lengths and end points.